Conventionally, as this type of electromagnetic flow meter there have been electromagnetic flow meters of a battery-driven type (hereinafter termed a “battery-type electromagnetic flow meter”). Instead of being supplied with a power supply through a commercial power supply, the battery-type electromagnetic flow meter is provided internally with a battery, where not only is the battery used as the power supply to drive the magnetic excitation circuit, but also to drive a flow rate measuring circuit that produces a flow rate measurement signal through a signal electromotive force that is produced between a pair of electrodes that are disposed facing each other within a measuring tube.
FIG. 6 illustrates the main components in a conventional battery-type electromagnetic flow meter (See, for example, Japanese Unexamined Patent Application Publication H9-126848 and Japanese Unexamined Patent Application Publication 2001-281029). In this FIG: 1 is a measuring tube; 2 is a magnetic excitation coil that is disposed with the direction in which the magnetic field is generated being perpendicular to the direction of flow of the fluid that flows within the measuring tube 1; 3 is a magnetic excitation circuit that provides a magnetic excitation electric current Iex (illustrated in FIG. 7) to the magnetic excitation coil 2 alternatingly in the positive direction and the negative direction, with non-magnetic-excitation periods interposed therebetween, provided with a positive magnetic excitation period and a negative magnetic excitation period before and after the non-magnetic-excitation period; 4a and 4b are a pair of detecting electrodes, disposed facing each other within the measuring tube 1, across the direction of flow of the fluid that flows within the measuring tube 1 and perpendicular to the direction of the magnetic field generated by the magnetic excitation coil 2; 5 is a ground electrode; 6 is a flow rate measuring circuit for detecting a signal electromotive force that is produced between the detecting electrodes 4a and 4b, and for outputting, as a flow rate measurement signal, a pulse signal with a duty ratio with a frequency that varies in accordance with the flow rate of the fluid that flows within the measuring tube 1, based on the detected signal electromotive force; and 7 is an internal battery. A power supply voltage VB is supplied from the internal battery 7 to the magnetic excitation circuit 3 and the flow rate measuring circuit 6.
In the battery-type electromagnetic flow meter, the magnetic excitation circuit 3 is provided with a magnetic excitation electric current direction switching circuit 3-1, a magnetic excitation electric current value adjusting circuit 3-2, and the like. The magnetic excitation electric current direction switching circuit 3-1 receives instructions from the flow rate measuring circuit 6 to switch the direction of the magnetic excitation electric current Iex to the magnetic excitation coil 2 alternatingly to the positive direction and the negative direction, with non-magnetic-excitation periods interposed therebetween. The magnetic excitation electric current value adjusting circuit 3-2 receives instructions from the flow rate measuring circuit 6 to adjust the value of the magnetic excitation electric current Iex to the magnetic excitation coil 2. The flow rate measuring circuit 6 is provided with a CPU 6-1. Instructions are issued from this CPU 6-1 to the magnetic excitation electric current direction switching circuit 3-1 and to the magnetic excitation electric current value adjusting circuit 3-2. Moreover, the flow rate is calculated by the CPU 6-1 based on the signal electromotive force produced between the detecting electrodes 4a and 4b. 
In this battery-type electromagnetic flow meter, the operating power supply depends on the internal battery 7, so if the internal battery 7 wears out, it is then necessary to replace with a new battery. Because of this, it is desirable to extend the period with which the batteries are replaced, so when providing the magnetic excitation electric current Iex to the magnetic excitation coil 2, the consumption of the electric power is reduced through the provision of the non-magnetic-excitation periods between the positive magnetic excitation period and the negative magnetic excitation period. This system wherein the non-magnetic-excitation period is provided between the positive magnetic excitation period and the negative magnetic excitation period, that is, this system wherein a positive magnetic excitation period and a negative magnetic excitation period are provided before and after a non-magnetic-excitation period, is known as “tri-state magnetic excitation.”
In Japanese Unexamined Patent Application Publication H3-144314 (“JP '314”), for this type of tri-state magnetic excitation electromagnetic flow meter, there are electromagnetic flow meters wherein it is possible to detect a fault that occurs when, for example, the center of the measuring tube empty, or where, for example, something that is electrically insulating becomes adhered to an electrode, where the non-magnetic-excitation period wherein a specific amount of time elapses from the commencement of the transition from the positive magnetic excitation period to the non-magnetic-excitation period is defined as a first period, and the non-magnetic-excitation period wherein a specific amount of time elapses from the commencement of the transition from the negative magnetic excitation period to the non-magnetic-excitation period is defined as a second period, and the voltage difference between the electrodes in the first period is calculated, the voltage difference between the electrodes in the second period is calculated, where a difference between the voltage difference between the electrodes during the first period and the voltage difference between the electrodes during the second period is calculated, and an evaluation that there is a fault is made if this difference exceeds a predetermined reference value.
That is, in the electromagnetic flow meter described in JP '314, if differential noise is produced at the time of switching the magnetic excitation periods and the inside of the measuring tube is empty, then the differential noise that is produced will be large due to the floating capacitance that is formed between the magnetic excitation coil and the electrode. This differential noise is detected as the voltage difference between the electrodes during the first period at the time of the transition from the positive magnetic excitation period to the non-magnetic-excitation period, and detected as the voltage difference between the electrodes in the second period at the time of the transition from the negative magnetic excitation period to the non-magnetic-excitation period, and the fault evaluation is performed based on the voltage differences that are detected between the electrodes. Note that because the polarity of the differential noise that is produced between the electrodes during the first period is opposite of the polarity of the differential noise that is produced between the electrodes during the second period, the differential noise that is produced between the electrodes is detected with twice the magnitude through calculating the difference between the voltage difference between the electrodes in the first period and the voltage difference between the electrodes in the second period.
However, in the tri-state magnetic excitation electromagnetic flow meter disclosed in JP '314, because the fault evaluation is performed based on the voltage differences between the electrodes, that is, in terms of the example illustrated in FIG. 6, because the fault evaluation is performed based on the difference between the voltage produced at the detecting electrode 4a and the voltage produced at the detecting electrode 4b, it is not possible to detect the differential noise except as a small value, and thus there is a problem in that the reliability of the fault evaluation is low.
The present invention is to solve this type of problem, and the object thereof is to provide an electromagnetic flow meter capable of increasing the reliability of the fault evaluation by detecting the differential noise as a large value.